Saturday, October 02, 2010

Here's another image of one of those braking induced fork failures that crop up from time to time. This was sent to me by a reader. The entire story of how the accident occured is mentioned on this blog.

Because of the lack of telescopic front suspensions like those nice mountain bikes have, rigid forks take the full brunt of a combination of two forces. One is the braking force that acts longitudinally backward to direction of motion but this has a component along the axis of the fork as well. The other is the force due to braking load transfer towards the front of the bike. This force acts inline with the fork axis. In essence, the two forces add together. I'm fairly certain that the quantity of this directed force along the fork is strongly dependent on the wheelbase of the bike and the rake angle of the fork. Lower wheelbases equate to more load transfer. Higher rake angles must also promote higher fork forces.

Undersized, thin walled tubes, such as forks, do not act kindly to hard braking forces. An example of this kind of buckling along with its physics was provided sometime back on my blog.

32 comments:

This is hilarious. Are you the same anonymous poster who's been trying to illicit a reply from me on my last post as well? I'll admit I'm a bit upset at Contador being linked to this issue. But I no longer have time for stupid doping gossip. Science of Sport did an elaborate writeup yesterday and today, so get your time's worth over there if you are not satisfied.

How much does the front suspension really soak up the fork load though? Certainly when you nose-in in a stop it's doing a fair bit but there still has to be a fairly large bending moment somewhere in the brake-fork-wheel system.

I'm guessing the fact that MTBs have huge diameter stancheons is more important than the actual suspension elements.

I've always felt a lot of road-style bikes with disks looked like the disk caliper braze on was just added to a more or less regular alloy fork and not modified to account for the load on the fork moving from right at or near the crown to near the blade tips.

In your opinion, is this a shortfall in the fork design standards, substandard workmanship, or user-induced damage coming home to roost? Fundamentally, if the pictured fork buckled, that was a static failure which is the easiest to avoid during design and qualification.

2. Do you think this happened from the braking forces alone? I read the cyclist's blog; this happened as she was avoiding a collision with another cyclist. She said she doesn't *think* she hit the other cyclist, but didn't sound 100% sure. What would you guess?

Lance = Hate Speach = Loss of Followers From The HiveOther dopers = Lets all get along ?? WTF ?

I used to enjoy following your great info, but after your hate mongering BS during the tour, I had enough of your spew. I guess you've decided to stick with what your good at and that is engineering, I applaud your change of attitude, maybe I will follow you again.

Fork flex is still an issue even with MTB crowd... MTB forks also use an offset at the crown instead of a rake near the axle.

On the road, the increased traction offered by asphalt and added weight of panniers makes it possible for more flex and heavier braking loads. I can flex the Kona p2 fork on the commuter with aggressive disc brake use. Though i'm not that concerned since it is not a custom performance job with "road dampening" thin-walled tubing.

That's definitely not a P2 fork, the drop outs aren't quite right and the fork crown is definitely wrong, looks like a Long Shen to me. Judging from the 105 cranks, brifters and titanium frame I'd say it's a disk brake road bike with custom forks. Ron, On the note of "exercising safely" my rucksack got caught on a tree branch at the weekend and I broke my shoulder blade which is very unusual apparently. If you're really bored I'd love to know how fast my back hit the floor, it's way beyond my ability to work it out myself. I was traveling at 15-17 mph, i ride a 26" wheel 19" mountain bike, i weigh 78 kilos, 5'10" tall and had about 8 kilos in my rucksack. No worries if you don't have time or inclination to do it, I'm just very, very bored and in quite some pain : O )

PS - I had a friend who does destruction testing for a living say he thought perhaps the steel had been heated too much when they attached the disc dropouts. Is that a possible contributing cause too?

The frame-maker is making me a new fork with fatter tubing. The diameter is the same, but the walls are thicker. Will thicker tubing actually prevent another failure of this type, or should they do something else to strengthen it?

Really glad you're ok Katie and it sounds like you'll be out and about very soon. In my very inexpert opinion it looks like fork blade wasn't thick or large enough to handle the disk forces, luckily it didn't sheer off, could have been worse. Hopefully it's not dented your confidence too much, ride safe, Matt

Your fork looks like a typical road fork with the tabs for the disc brake added on. The forces resolve much differently in a disc brake fork than in a caliper or cantliever brake fork. My recommedation is that your frame builder incorporate a larger diameter tube rather than just a thicker walled tube.

If you look at purpose built forks for disc brakes (like some cyclocross forks) you will see that they are designed differently. Most have have much larger diameter tubing the full length of the fork blade all the way down to the drop out. You will lose some of the natural dampening and shock absorption qualities of the fork becasue it will be overall much stiffer.

Heating related to brazing or welding the tab onto the fork blade could certainly be an issue in the strength of the tube. I would be less inclined to think this is an issue if the fork was built by a reputable frame builder. Those guys are usually pretty good with a torch, and understand the effects of heat on tubing fairly well.

I said that the design of a road fork to use disc brakes is different than the design of the classical road fork design with the curved fork blades. It was more likely a mechanical failure due to the tubing size at the brake mounting tabs than a heat treating issue. See the forks below as a typical design of a purpose built disc brake fork for the road. I did give the frame builder the benefit of the doubt to know the effects of heat on tubing strength.

See the link for an example:http://www.bikeman.com/FK2407.html

I run both cromoly and carbon fiber forks with disc brakes on my two mostly road going bikes, and have had no problems over the years. Any frame builder should do analysis before adapting a design to a new piece of equipment. I disagree in part with Ron's analysis above and believe it is the concentration of a bending moment at the point of failure specific to a disc brake application on Katie Ferguson's fork that caused it to fail.

The direction her frame builder is going is unclear from her post, where she says the tubing is fatter with the same diameter. I am confused, like I gained 20 pounds but still fit into the same pant size. She mentions thicker tubing wall. If she means the tube diameter will stay the same and the walls will be thicker, I fear she may have another failure in her future, a larger diameter tubing must be used, tube wall thickness plays a much smaller role in strength than diameter.

First, Federal Standards should be considered as minimum requirments to protect the saftey of the consumer. Some of thhe best at this time for bicycles components are EN, & DIN+. If tested in EN, or DIN+, this fork would have likely not have passed and so the Seattle 'Joe Blow's Custom Frame' Co. would have some actual data to improve the design BEFORE the product is sold and used by the customer. That is the entire pourpose why France requires all bicycle component sold to pass EN as a way to protect the customer.

Second, ISO 9000 has almost nothing to do with the Engineering, or Design of product, but instead is used as a frame work to create a Quality Manufacturing System that will ensure the product is created in a stable and repeatable manner.

So, to answer your question, YES I do want what I purchase manufactured in a manner that uses the essential structure of ISO 9000.

Just so you can clearly understand the point I've been getting at, I'll simplify down for you -

I don't want people to die. If some guy up in Seattle wants to make a product such as this, then they should accept that respinsibility and create product in a manner that ensures people won't die.

I am extremely troubled at the double standard some of you seem to have.

Wow people. There are bike design standards. You can, in fact see a list with links to/of some of them on the left hand side bar of this very page as you type your responses.

In particular, Ron was kind enough to link to ASTM F2273-03: Standard Test Methods for Bicycle Forks. Ron, are there any specifics in there for disk tabbed forks? I don't feel like paying for a copy of those to check it out but maybe you have one.

Just so I am clear.If the fork is sold in France, it is required by law to pass EN Standards.

If the Fork is sold in the US, there is no real requirements for it to meet.

In either case, you feel there should be no expectation that the manufacture has any moral, or ethical position to make a product that will perform as it is promoted to do and not possibly kill the customer?

Customers should assume that any manufacture, even hipster bike dudes in Seattle, do have any moral, or ethical standards that they use during the development and manufacturing of product?

In the case of local buckling, my understanding is that buckling strength increases faster with wall thickness than tubing diameter, FEA modeling of steel tubes show this.

Tubing manufacturers are always looking to making thinner-walled tubing, of increasing diameter, to increase AMoI. They stop when the buckling limits start to cancel out the projected increases in strength due to improved AMoI. If higher AMoI actually increased buckling strength, tubing diameters would only be limited by mechanical clearance considerations and aerodynamic drag, but I don't see that on real bikes.

Sheldon is not "missing the math", IMO his claims are based on practical experience.

Great discussion going on. Here's a bit of engineering convention regarding failure modes and stress, etc. Buckling usually refers to a failure mode that starts as an elastic response to a load. Elastic means that the material hasn't yielded and will return to it's original configuration when the load is removed. Buckling is often followed by yielding, in which case the material stays deformed after the load is released. The failed fork is undoubtedly yielded, but it is unlikely the result of buckling. Buckling usually only happens in very long and skinny structures, more like seat stays. This failure is likely due to stress concentration and material degradation from brazing or welding. BTW, I'm not a P.E., just a practicing engineer for the last 40 years. -LaLa